Catalytic distillation (CD) combines catalytic reaction and separation in a single distillation unit. This idea was first implemented early in the 1920s for the production of esters (Backhaus, 1921) and has been applied to a number of chemical processes based on homogeneous catalysts. The advantages of combining reaction and separation were not fully appreciated until 1980, when Smith patented a new catalytic distillation technology using heterogeneous catalysts (Smith, 1980).
Conventional chemical processes that utilise a distillation process (non-catalytic) mainly consist of two separate unit operations. These include a unit hosting chemical reactions and another unit for separating the different components from the resulting reaction mixture. Under such circumstances, it is difficult to recycle the heat produced by the chemical reaction, and cooling is often needed to control the temperature in the reaction zone, thus resulting in ineffective energy utilization in the process. In addition, the productivity of a preferred compound is often limited by the conversion and selectivity in a chemical process due to equilibrium limitations. Since heat and mass transfer resistance are common problems in such a reaction unit, poor catalytic performance may occur together with a shorter catalyst lifetime.
In order to avoid the chemical equilibrium limitation and make full use of reaction heat, the simple combination of a chemical reaction unit with the separation unit in a traditional distillation column has provided a successful approach for a number of catalytic reaction processes. This combination was first utilized for homogeneous reaction systems. Because these systems involve both reaction and distillation, the name reactive distillation (RD) was coined for these processes. The traditional RD processes were mainly based on homogeneous reaction systems; thus, RD is also referred to as homogeneous catalytic distillation. Although RD processes often result in both high reaction rates and high selectivity to certain desired products, several disadvantages still prevail. These include separation of catalyst from the reaction products, recovery of catalyst, column fouling and corrosion. In addition, if the product purity with respect to the catalyst composition is strictly necessary, the products have to be intensively treated after reactive distillation to ensure a satisfactory level of catalyst removal, which can increase operating costs.
For gas and/or liquid reactions occurring on the surface of solid catalysts (heterogeneous systems), reaction products can be easily separated from the catalyst system. If a heterogeneous reaction can be managed within a distillation unit, the difficulty of separation encountered in homogeneous catalytic distillation can be overcome. However, another factor that must be addressed is to ensure that sufficient catalyst is placed into the column without significantly increasing the pressure drop. It was not until 1980 that Smith (1980) patented a method of suspending catalyst pellets inside a distillation column using fibreglass containment bags, which are known as Texas teabags. Use of these bags permits the use of heterogeneous catalysts without giving rise to large increases in pressure drop. In contrast to homogeneous catalytic distillation, heterogeneous catalytic distillation is preferentially termed catalytic distillation (CD).
In the CD process, the solid catalyst has to be packed in a suitable manner inside the distillation column so as to maximize contact between vapour and liquid phases, but to minimize column flooding. Indeed, various methods have been reported for supporting or containing catalysts [Crossland et al., U.S. Pat. No. 5,431,890; Hearn, U.S. Pat. No. 5,266,546; Shelden, U.S. Pat. No. 5,417,938]. It should be noted that with all these methods, the catalyst is enclosed inside a device which can increase the mass transfer resistance of the liquid and gas phases in the column.
Since a catalytic distillation process combines heterogeneous reaction and separation in a single distillation column, the following advantages can often be obtained over conventional fixed bed reactors:
i) The capital and production costs are reduced because two operations are combined in a single unit.
ii) The energy consumption can be minimized as the heat of the reaction is used for the in-situ vaporization of the reactants.
iii) The conversion of the reactant can be enhanced through internal recycling.
iv) As the reaction products are continuously removed from the reaction site or the surface of catalyst as they are formed, the normal chemical equilibrium limitation does not apply, thus allowing a higher conversion to be achieved.
v) The selectivity for a desired product can also be improved by continuously removing of the product away from the reaction site or the catalyst surface as the product is formed.
vi) The heat generated by the reaction can be efficiently carried away by the liquid and vapour, thus eliminating or avoiding the formation of hot spots on or in the vicinity of the catalytic site. This movement of the flushing liquid or vapour has a flushing effect in that it removes the higher molecular weight oligomerization products down from the reaction zone, and thereby freeing the catalytic site for another reaction. The possible fouling and poisoning of the catalyst are greatly reduced.vi) The catalyst lifetime can also be improved because the catalyst bed is surrounded by hot or boiling liquid and vapour that constantly exert a flushing on the catalyst site. Such a flushing effect remove products and by products that can undergo further reaction which can lead to possible fouling and poising of the catalyst.
Although there are many advantages for CD technology over the conventional processes mentioned above, catalytic distillation is not suitable for use in all chemical reaction process. In order to achieve the benefits from a CD process, a chemical reaction system should preferably satisfy the following requirements:
i) The reaction should take place in the liquid phase.
ii) The catalyst should be heterogeneous and stable thermally as well as chemically and physically to retain its structural integrity for maintenance of a long lifetime.
iii) The reaction should be exothermic and in situations where the reaction is equilibrium limited, the CD process presented the option to shift that equilibrium more to the right to achieve higher conversion and higher productivity much more efficiently.
One chemical reaction that satisfies these requirements is the oligomerization of lower alkenes (alkene molecules having from 2 to 6 carbon atoms). Alkylation and oligomerization of lower alkenes was first disclosed by Huss and Kennedy (1990) and Smith et al. (1991). The oligomerization of lower alkenes is an important industrial reaction and represents a route to the production of intermediates used for the production of motor fuels, plasticizers, pharmaceuticals, dyes, resins, detergents, lubricants and additives (O'Connor and Kojima, 1990). With respect to butene oligomerization, the less branched dimer products, octenes, are particularly useful in the manufacture of plasticizers. If heavily branched, the mixture can be used as a gasoline blender.
Historically, the exploitation of all of the C4 fractions obtained as by-product from hydrocarbon fluid catalytic cracking and steam cracking to produce high value products (high octane value product) has been lacking. Butadiene, a component of the by-product is useful for rubber production and is extracted from the by-product, leaving the remaining C4 fractions as a mixture referred to as Raffinate I. The isobutene that is contained in Raffinate I was used as a source for production of methyl tert-butyl ether (MTBE). The remaining components of the C4 fractions after the removal of isobutene, consisting mainly of linear C4 hydrocarbon (butenes), was mainly used as a gasoline blender, albeit a poor one. In certain cases, this product was simply disposed of by flaring. In Raffinate II, n-butene is present at an average content from 70% to 80% and in some cases can be in the ninety percentage ranges. Using this resource, smaller oligomers, particularly C8 and C12, are being produced by current catalytic oligomerization processes.
A variety of butene oligomerization processes have been proposed (Keim et al., 1979; Mathys, 1984; Beltrame et al., 1994) based on homogeneous and heterogeneous reactions. These processes are exclusively focused on the catalyst selection and process optimization so that a high oligomerization rate with a high selectivity to desired products, mainly short and less branched oligomers, can be obtained.
The use of catalytic distillation to enhance the oligomerization of alkenes was first disclosed in U.S. Pat. No. 5,003,124 to Smith in 1991. This process utilized an acidic ion exchange resin placed inside a fibre glass bag.
Further research has been carried out in the field of alkene oligomerization, but in most cases, the oligomerization catalyst is contained within a second structure such as a cloth or mesh bag, and the reactants have to pass through this structure to access the catalyst. Likewise, the products have to pass through the structure to be removed away from the catalyst. In one such example, Podrebarac (1992) studied butene dimerization in a CD column using a nickel exchanged zeolite catalyst, where the zeolite was placed directly in fibreglass bags. The zeolite catalyst in this case was quickly deactivated by the production of undesirable long chain oligomers, which oligomers blocked the active sites on the catalyst. The system also displayed poor selectivity to octane due to the mass transfer resistance caused by the fibreglass bags.
Additional work has been carried out to find alternative methods of placing the catalyst directly in the reactive zone of the distillation column, without resorting to the containment of the catalyst in secondary structures such as cloth or mesh bags.
U.S. Pat. No. 6,291,719 B1 to Gao et al. discloses dual-functional catalyst structures having very specific shapes. These catalyst structures, which are formed from a resin catalyst, a metal oxide superacid catalyst or a molecular sieve catalyst, are shown to be suitable for etherification reactions, alkylation reactions, hydrogenation reactions and for the decomposition of MTBE. U.S. Pat. No. 6,291,719 B1 discloses catalyst structures which are limited to two very specific shapes, and the materials used also possess markedly low surface area values.
U.S. Pat. No. 5,244,929 to Gottlieb et al. also discloses moulded organic catalyst bodies made of strongly acid or basic ion exchange resins.